Sir Vinny . . . . . .

Ye olde Knob:

And with the unit old enough to be hopefully not getting the later influx of Japonie knobs being supplied and their metric screws, and the threading hopefully being not the first 6-32 . . not beefy enough, along with their tips provided surfactal area . . . but very probably a 8-32 . . .but a 10-32 would be getting on towards the probable max size.

(Forbid that its not a 10-24)

Now the next fallacy, in the probability of the upping from a found 8-32 up to a 10-32, is that finite short distance between the hole and the opposite end of that 1/4 in hole in the brass? insets collar.
The conventional tap which you are going to find at the Home Despot or Lowes(T) is going to have you then finding that the gradual chamfering of the initial starting part of the tap will have you bottoming out against the opposite wall of the collar.
That being EVEN before you have established a good chasing out of the smaller threading size and re established into a perfected reforming to the NEW thread size . . . THROUGHOUT its length.

The alternative ?.. The journeyman machinist would merely reach in his tool drawer and pull out that which is known as a chaser tap, which is one that has no LONG GRADUAL TAPERING of the threading onset, but merely a very slight chamfer on the first 2 or so threads . It would then establish a conforming threading the whole lenghth of the collar and still not be bottoming out against the opposite collar wall.
In my case I have made several of these from the remnants of past times broken off taps, by the use of a Dremel tool and a cut off wheel.

What wud I does ?

I would figure on your trip to the hardware store and getting an 1/8 hi speeeeed drill bit and both a 6-32 and an 8-32 tap.( That 1/8 size is a bit larger than the specified numerical size for that taps use but it is cheeeper and more commonly found AND the final chasing out to upsizing to the 8-32 size will have then left a nice conformal fit.

Then the old set screw is removed and the drill held in the best alignment to pass the drill bit thru the old "wallered" out hole and drill a hole thru the inset 1/4 in collars OPPOSITE wall and on out the opposite side of the knobs plastic /or /bakelite surround.

Then you tap thru the old hole and initially start and finally finish up by passing out the NEWLY drilled hole.
Then the 8-32 top will easily start in the existant 6-32 threading and establish a VERY conformal threading to the final sizing, with no end play limitation due to the old hole letting the tap end travel into that old open area.

Then install a set screw.

Hint:

Lubricant on the tap a must and once the initial 2 turns are established for a start off "bite", then the sequential 1/2 a turn of tapping and then back up 3/4 turn,for chasing then forward to get a fresh 1/2 turn then 3/4 turn back . . .etc,etc. over and over.

Hard to remember the last time I even had a tap sieze or had one break on me since doing that back up sequence, particularly with steel or a stainless steel block being the real challenge, your non ferrous metals . . . are a snap.

A Shiny New Pointer:

NOW the way I used to make my own custom pointers for my custom communications / ham receivers.
Very much akin to that Heath knob of yours, cept I opted for a narrow long rectangle with the corners rounded off. Yours . . . instead . . . is being a gradually tapered to the tip pointer.

I'm expecting the rear of the knob to be having 3 small screws affixing the pointer to the knob back. Get the pointer free and figure the dimensions of the needed replacement piece of plastic.

Then the hunt is on for the replacement plastic, now I would think that the MINIMUM size in thickness would be about two thickness of playing cards, as you know how stable that even 1 thickness is in holding its shape. On the other end of the thickness spectrum might be up to an 1/8 of an inch. Then you scour the house / garage/shop to then see what you have that might fulfill the supplying of that piece of clear plastic.

Or walk the "Dollar" - Thrift store aisles to see what found obscure item just might additionally fulfill YOUR needs in magnificently providing a piece of plastic of that size.

Then . . . going full list price mode . . .I know that there are plastic picture frame "glass" inserts. . . . but possibly overpriced for its plastics scrap value.

Also . . . Dritz pattern maker template for the home seamstress . . highly overpriced at $4.95 for its plastic.
Check Homeless Despot or Lowes(T) for a small Plexiglass cut off scrap . . for free . . in their cut off bin . . . BUT, avoid softer Lexar as its quite a bear to refinish a cut edge on . . .with the most successfull technique being the multiple drags of an almost vertical box cutters sharp blade in a scraping action of taking away curls of plastic. PLUS it also has an offensive subtle blue coloration to it.

If using Plexiglass, a sheet of wetted # 400 wet and dry sandpaper on a flat surface and then working the cut edges of the created plastic pointer across it will finally yield a matte finish.

Then, if a perfectly clear finish is needed from that matte surface, the soaking of a sheet of paper with Brasso and working on the same flat surface, will eventually give a pristine, glass clear finish on the Plexiglass edges.
The centered HAIRLINE mark is made by initially marking in its extreme end limits with a micro tip Blue Sharpie on the BACK side of your plastic pointer. Then double check to see that it is centered. Then at the NARROW dimension of the pointer end, you might need a scrap of that plastic to the side to fully support a wooden/stainless steel(cork backed) ruler that you will line up beside the alignment marks.

The cut/scribe will be done with an Exacto knife with a fresh #11 blade mounted in it, but you will be using it BACKWARDS as a vee scratching tool, with it held about 30 degrees down from the vertical.

Make a trial movement the whole length between the two indexing marks, with out touching the plastic and then touch lightly to make an initial slight scratching the length of then a few more runs to develop just a bit of a progressively deeper vee groove in the plastic. . . but it doesn't need to be excessively deep !

Then the groove gets filled in with your black coloring agent, thru the times I have used a black china marker(aka price marker, grease pencil ?), black crayola, India ink, Ki Wi shoe polish (solid . .not liquid), enamel paint in a can. Needless to say any Sharpie perm types of markers are out, since they tend to leave a subtle stain on any overcreep at the side of the line which mineral spirits / lighter fluid /naptha/ "409" won't quite take out. While, more agressive clean up solvents are going to react ADVERSELY with your plastic.

(Inclusive of MEK, toluene, xylene, acetone, methyl chloride, lacquer thinner and bannana oil, etc / what ? else.)

Thassit . . . .did you get the point(er) ?

73's de Edd